Temperature stable solid electrolytic capacitor

ABSTRACT

A capacitor whose electrical properties can be stable under a variety of different conditions is provided. The solid electrolyte of the capacitor is formed from a combination of an in situ polymerized conductive polymer and a hydroxy-functional nonionic polymer. One benefit of such an in situ polymerized conductive polymer is that it does not require the use of polymeric counterions (e.g., polystyrenesulfonic anion) to compensate for charge, as with conventional particle dispersions, which tend to result in ionic polarization and instable electrical properties, particularly at the low temperatures noted above. Further, it is believed that hydroxy-functional nonionic polymers can improve the degree of contact between the polymer and the surface of the internal dielectric, which unexpectedly increases the capacitance performance and reduces ESR.

RELATED APPLICATIONS

The present application claims priority to U.S. Provisional ApplicationSer. No. 61/673,412, filed on Jul. 19, 2012, which is incorporatedherein in its entirety by reference thereto.

BACKGROUND OF THE INVENTION

Solid electrolytic capacitors (e.g., tantalum capacitors) are typicallymade by pressing a metal powder (e.g., tantalum) around a metal leadwire, sintering the pressed part, anodizing the sintered anode, andthereafter applying a solid electrolyte. Intrinsically conductivepolymers are often employed as the solid electrolyte due to theiradvantageous low equivalent series resistance (“ESR”) and“non-burning/non-ignition” failure mode. Recently, conductive polymerslurries that contain a complex of poly(3,4-ethylenedioxythiophene) andpolystyrenesulfonic acid (“PEDT:PSS complex”) have been employed as asolid electrolyte material due to their ability to handle high voltages,such as experienced during a fast switch on or operational currentspike. While some benefits have been achieved, one problem with polymerslurry-based capacitors is that their capacitance is highly temperaturedependent. For example, the capacitance tends to drop significantly atlow temperatures (e.g., −55° C.), which can prevent the use of suchcapacitors in cold temperature environments, such as often experiencedin aerospace or military applications.

As such, a need currently exists for a solid electrolytic capacitorhaving improved stability at a wide variety of different temperatures.

SUMMARY OF THE INVENTION

In accordance with one embodiment of the present invention, a solidelectrolytic capacitor is disclosed that comprises a sintered porousanode, a dielectric layer that overlies the anode body, and a solidelectrolyte overlying the dielectric layer. The solid electrolytecomprises an in situ polymerized conductive polymer and ahydroxy-functional nonionic polymer.

In accordance with another embodiment of the present invention, a methodfor forming a solid electrolytic capacitor is disclosed that comprisesanodically oxidizing a sintered porous anode to form a dielectric layerthat overlies the anode and forming a solid electrolyte over thedielectric layer by a process that includes chemically polymerizing amonomer in situ to form a conductive polymer, and thereafter, applying ahydroxy-functional nonionic polymer.

Other features and aspects of the present invention are set forth ingreater detail below.

BRIEF DESCRIPTION OF THE DRAWING

A full and enabling disclosure of the present invention, including thebest mode thereof, directed to one of ordinary skill in the art, is setforth more particularly in the remainder of the specification, whichmakes reference to the appended FIGURE in which:

FIG. 1 is a schematic illustration of one embodiment of a capacitor thatmay be formed in accordance with the present invention.

Repeat use of references characters in the present specification anddrawing is intended to represent same or analogous features or elementsof the invention.

DETAILED DESCRIPTION OF REPRESENTATIVE EMBODIMENTS

It is to be understood by one of ordinary skill in the art that thepresent discussion is a description of exemplary embodiments only, andis not intended as limiting the broader aspects of the presentinvention, which broader aspects are embodied in the exemplaryconstruction.

Generally speaking, the present invention is directed to a capacitorwhose electrical properties can be stable under a variety of differentconditions. For example, the capacitance and/or equivalent seriesresistance (“ESR”) of the capacitor may remain relatively stable at lowtemperatures, such as about 25° C. or less, in some embodiments about10° C. or less, in some embodiments about 0° C. or less, and in someembodiments, from about −75° C. to about −25° C. (e.g., −55° C.), aswell as at a wide range of frequencies, such from about 10 Hz to about100 kHz. Even under such conditions, the capacitance may still berelatively high. For example, the capacitor can exhibit a relativelyhigh percentage of its wet capacitance, which enables it to have only asmall capacitance loss and/or fluctuation in the presence of atmospherehumidity. This performance characteristic is quantified by the“wet-to-dry capacitance percentage”, which is determined by theequation:Wet-to-Dry Capacitance=(Dry Capacitance/Wet Capacitance)×100

The capacitor of the present invention, for instance, may exhibit awet-to-dry capacitance percentage of about 50% or more, in someembodiments about 60% or more, in some embodiments about 70% or more,and in some embodiments, from about 80% to 100%. In addition to a stablecapacitance, the capacitor may also maintain a low equivalence seriesresistance (“ESR”) under the conditions noted above, such as less thanabout 100 mohms, in some embodiments less than about 75 mohms, in someembodiments from about 0.01 to about 60 mohms, and in some embodiments,from about 0.05 to about 50 mohms, measured at an operating frequency of100 kHz.

The present inventors have discovered that the ability to achieve suchgood electrical performance can be achieved through a unique andcontrolled combination of features relating, at least in part, to thesolid electrolyte. More particularly, the solid electrolyte is formedfrom a combination of an in situ polymerized conductive polymer and ahydroxy-functional nonionic polymer. One benefit of such an in situpolymerized conductive polymer is that it does not require the use ofpolymeric counterions (e.g., polystyrenesulfonic anion) to compensatefor charge, as with conventional particle dispersions, which tend toresult in ionic polarization and instable electrical properties,particularly at the low temperatures noted above. Further, it isbelieved that hydroxy-functional nonionic polymers can improve thedegree of contact between the polymer and the surface of the internaldielectric, which unexpectedly increases the capacitance performance andreduces ESR.

Various embodiments of the present invention will now be described inmore detail.

I. Anode

The anode is formed from a valve metal composition. The specific chargeof the composition may vary, such as from about 5,000 μF*V/g to about250,000 μF*V/g, in some embodiments from about 25,000 μF*V/g to about200,000 μF*V/g or more, and in some embodiments, from about 50,000 toabout 150,000 μF*V/g. As is known in the art, the specific charge may bedetermined by multiplying capacitance by the anodizing voltage employed,and then dividing this product by the weight of the anodized electrodebody.

The valve metal composition generally contains a valve metal (i.e.,metal that is capable of oxidation) or valve metal-based compound, suchas tantalum, niobium, aluminum, hafnium, titanium, alloys thereof,oxides thereof, nitrides thereof, and so forth. For example, the valvemetal composition may contain an electrically conductive oxide ofniobium, such as niobium oxide having an atomic ratio of niobium tooxygen of 1:1.0±1.0, in some embodiments 1:1.0±0.3, in some embodiments1:1.0±0.1, and in some embodiments, 1:1.0±0.05. The niobium oxide may beNbO_(0.7), NbO_(1.0), NbO_(1.1), and NbO₂. Examples of such valve metaloxides are described in U.S. Pat. No. 6,322,912 to Fife; U.S. Pat. No.6,391,275 to Fife et al.; U.S. Pat. No. 6,416,730 to Fife et al.; U.S.Pat. No. 6,527,937 to Fife; U.S. Pat. No. 6,576,099 to Kimmel, et al.;U.S. Pat. No. 6,592,740 to Fife, et al.; and U.S. Pat. No. 6,639,787 toKimmel, et al.; and U.S. Pat. No. 7,220,397 to Kimmel, et al., as wellas U.S. Patent Application Publication Nos. 2005/0019581 to Schnitter;2005/0103638 to Schnitter, et al.; 2005/0013765 to Thomas, et al.

To form the anode, a powder of the valve metal composition is generallyemployed. The powder may contain particles any of a variety of shapes,such as nodular, angular, flake, etc., as well as mixtures thereof. Thebulk density (also known as Scott density) of the powder may be fromabout 0.1 to about 2 grams per cubic centimeter (g/cm³), in someembodiments from about 0.2 g/cm³ to about 1.5 g/cm³, and in someembodiments, from about 0.4 g/cm³ to about 1 g/cm³. “Bulk density” maybe determined using a flow meter funnel and density cup. Morespecifically, the powder sample may be poured through the funnel intothe cup until the sample completely fills and overflows the periphery ofthe cup, and thereafter sample may be leveled-off by a spatula, withoutjarring, so that it is flush with the top of the cup. The leveled sampleis transferred to a balance and weighed to the nearest 0.1 gram todetermine the density value. Such an apparatus is commercially availablefrom Alcan Aluminum Corp. of Elizabeth, N.J. The particles may also havean average size (e.g., width) of from about 0.1 to about 100micrometers, in some embodiments from about 0.5 to about 70 micrometers,and in some embodiments, from about 1 to about 50 micrometers.

To facilitate the construction of the anode, certain additionalcomponents may also be included in the powder. For example, the powdermay be optionally mixed with a binder and/or lubricant to ensure thatthe particles adequately adhere to each other when pressed to form theanode body. Suitable binders may include, for instance, poly(vinylbutyral); poly(vinyl acetate); poly(vinyl alcohol); poly(vinylpyrrolidone); cellulosic polymers, such as carboxymethylcellulose,methyl cellulose, ethyl cellulose, hydroxyethyl cellulose, andmethylhydroxyethyl cellulose; atactic polypropylene, polyethylene;polyethylene glycol (e.g., Carbowax from Dow Chemical Co.); polystyrene,poly(butadiene/styrene); polyamides, polyimides, and polyacrylamides,high molecular weight polyethers; copolymers of ethylene oxide andpropylene oxide; fluoropolymers, such as polytetrafluoroethylene,polyvinylidene fluoride, and fluoro-olefin copolymers; acrylic polymers,such as sodium polyacrylate, poly(lower alkyl acrylates), poly(loweralkyl methacrylates) and copolymers of lower alkyl acrylates andmethacrylates; and fatty acids and waxes, such as stearic and othersoapy fatty acids, vegetable wax, microwaxes (purified paraffins), etc.The binder may be dissolved and dispersed in a solvent. Exemplarysolvents may include water, alcohols, and so forth. When utilized, thepercentage of binders and/or lubricants may vary from about 0.1% toabout 8% by weight of the total mass. It should be understood, however,that binders and/or lubricants are not necessarily required in thepresent invention.

The resulting powder may then be compacted to form a pellet using anyconventional powder press device. For example, a press mold may beemployed that is a single station compaction press containing a die andone or multiple punches. Alternatively, anvil-type compaction pressmolds may be used that use only a die and single lower punch. Singlestation compaction press molds are available in several basic types,such as cam, toggle/knuckle and eccentric/crank presses with varyingcapabilities, such as single action, double action, floating die,movable platen, opposed ram, screw, impact, hot pressing, coining orsizing. The powder may be compacted around an anode lead wire. The wiremay be formed from any electrically conductive material, such astantalum, niobium, aluminum, hafnium, titanium, etc., as well aselectrically conductive oxides and/or nitrides of thereof.

After compaction, the resulting anode body may then be diced into anydesired shape, such as square, rectangle, circle, oval, triangle,hexagon, octagon, heptagon, pentagon, etc. The anode body may also havea “fluted” shape in that it contains one or more furrows, grooves,depressions, or indentations to increase the surface to volume ratio tominimize ESR and extend the frequency response of the capacitance. Theanode body may then be subjected to a heating step in which most, if notall, of any binder/lubricant are removed. For example, the anode body istypically heated by an oven that operates at a temperature of from about150° C. to about 500° C. Alternatively, the binder/lubricant may also beremoved by contacting the pellet with an aqueous solution, such asdescribed in U.S. Pat. No. 6,197,252 to Bishop, et al.

Thereafter, the pellet is sintered to form a porous, integral mass. Thetemperature, atmosphere, and time of the sintering may depend on avariety of factors, such as the type of anode, the size of the anode,etc. Typically, sintering occurs at a temperature of from about fromabout 800° C. to about 1900° C., in some embodiments from about 1000° C.to about 1500° C., and in some embodiments, from about 1100° C. to about1400° C., for a time of from about 5 minutes to about 100 minutes, andin some embodiments, from about 30 minutes to about 60 minutes. Ifdesired, sintering may occur in an atmosphere that limits the transferof oxygen atoms to the anode. For example, sintering may occur in areducing atmosphere, such as in a vacuum, inert gas, hydrogen, etc. Thereducing atmosphere may be at a pressure of from about 10 Torr to about2000 Torr, in some embodiments from about 100 Torr to about 1000 Torr,and in some embodiments, from about 100 Torr to about 930 Torr. Mixturesof hydrogen and other gases (e.g., argon or nitrogen) may also beemployed.

The anode may also have a relatively low carbon and oxygen content. Forexample, the anode may have no more than about 50 ppm carbon, and insome embodiments, no more than about 10 ppm carbon. Likewise, the anodemay have no more than about 3500 ppm oxygen, in some embodiments no morethan about 3000 ppm oxygen, and in some embodiments, from about 500 toabout 2500 ppm oxygen. Oxygen content may be measured by LECO OxygenAnalyzer and includes oxygen in natural oxide on the tantalum surfaceand bulk oxygen in the tantalum particles. Bulk oxygen content iscontrolled by period of crystalline lattice of tantalum, which isincreasing linearly with increasing oxygen content in tantalum until thesolubility limit is achieved. This method was described in “CriticalOxygen Content In Porous Anodes Of Solid Tantalum Capacitors”,Pozdeev-Freeman et al., Journal of Materials Science: Materials InElectronics 9, (1998) 309-311 wherein X-ray diffraction analysis (XRDA)was employed to measure period of crystalline lattice of tantalum.Oxygen in sintered tantalum anodes may be limited to thin naturalsurface oxide, while the bulk of tantalum is practically free of oxygen.

As noted above, an anode lead may also be connected to the anode bodythat extends in a longitudinal direction therefrom. The anode lead maybe in the form of a wire, sheet, etc., and may be formed from a valvemetal compound, such as tantalum, niobium, niobium oxide, etc.Connection of the lead may be accomplished using known techniques, suchas by welding the lead to the body or embedding it within the anode bodyduring formation (e.g., prior to compaction and/or sintering).

II. Dielectric

The anode is also coated with a dielectric. The dielectric may be formedby anodically oxidizing (“anodizing”) the sintered anode so that adielectric layer is formed over and/or within the anode. For example, atantalum (Ta) anode may be anodized to tantalum pentoxide (Ta₂O₅).Typically, anodization is performed by initially applying a solution tothe anode, such as by dipping anode into the electrolyte. A solvent isgenerally employed, such as water (e.g., deionized water). To enhanceionic conductivity, a compound may be employed that is capable ofdissociating in the solvent to form ions. Examples of such compoundsinclude, for instance, acids, such as described below with respect tothe electrolyte. For example, an acid (e.g., phosphoric acid) mayconstitute from about 0.01 wt. % to about 5 wt. %, in some embodimentsfrom about 0.05 wt. % to about 0.8 wt. %, and in some embodiments, fromabout 0.1 wt. % to about 0.5 wt. % of the anodizing solution. Ifdesired, blends of acids may also be employed.

A current is passed through the anodizing solution to form thedielectric layer. The value of the formation voltage manages thethickness of the dielectric layer. For example, the power supply may beinitially set up at a galvanostatic mode until the required voltage isreached. Thereafter, the power supply may be switched to apotentiostatic mode to ensure that the desired dielectric thickness isformed over the entire surface of the anode. Of course, other knownmethods may also be employed, such as pulse or step potentiostaticmethods. The voltage at which anodic oxidation occurs typically rangesfrom about 4 to about 250 V, and in some embodiments, from about 9 toabout 200 V, and in some embodiments, from about 20 to about 150 V.During oxidation, the anodizing solution can be kept at an elevatedtemperature, such as about 30° C. or more, in some embodiments fromabout 40° C. to about 200° C., and in some embodiments, from about 50°C. to about 100° C. Anodic oxidation can also be done at ambienttemperature or lower. The resulting dielectric layer may be formed on asurface of the anode and within its pores.

Although not required, in certain embodiments, the dielectric layer maypossess a differential thickness throughout the anode in that itpossesses a first portion that overlies an external surface of the anodeand a second portion that overlies an interior surface of the anode. Insuch embodiments, the first portion is selectively formed so that itsthickness is greater than that of the second portion. It should beunderstood, however, that the thickness of the dielectric layer need notbe uniform within a particular region. Certain portions of thedielectric layer adjacent to the external surface may, for example,actually be thinner than certain portions of the layer at the interiorsurface, and vice versa. Nevertheless, the dielectric layer may beformed such that at least a portion of the layer at the external surfacehas a greater thickness than at least a portion at the interior surface.Although the exact difference in these thicknesses may vary depending onthe particular application, the ratio of the thickness of the firstportion to the thickness of the second portion is typically from about1.2 to about 40, in some embodiments from about 1.5 to about 25, and insome embodiments, from about 2 to about 20.

To form a dielectric layer having a differential thickness, amulti-stage process is generally employed. In each stage of the process,the sintered anode is anodically oxidized (“anodized”) to form adielectric layer (e.g., tantalum pentoxide). During the first stage ofanodization, a relatively small forming voltage is typically employed toensure that the desired dielectric thickness is achieved for the innerregion, such as forming voltages ranging from about 1 to about 90 volts,in some embodiments from about 2 to about 50 volts, and in someembodiments, from about 5 to about 40 volts. Thereafter, the sinteredbody may then be anodically oxidized in a second stage of the process toincrease the thickness of the dielectric to the desired level. This isgenerally accomplished by anodizing in an electrolyte at a highervoltage than employed during the first stage, such as at formingvoltages ranging from about 50 to about 350 volts, in some embodimentsfrom about 60 to about 300 volts, and in some embodiments, from about 70to about 200 volts. During the first and/or second stages, theelectrolyte may be kept at a temperature within the range of from about15° C. to about 95° C., in some embodiments from about 20° C. to about90° C., and in some embodiments, from about 25° C. to about 85° C.

The electrolytes employed during the first and second stages of theanodization process may be the same or different. Typically, however, itis desired to employ different solutions to help better facilitate theattainment of a higher thickness at the outer portions of the dielectriclayer. For example, it may be desired that the electrolyte employed inthe second stage has a lower ionic conductivity than the electrolyteemployed in the first stage to prevent a significant amount of oxidefilm from forming on the internal surface of anode. In this regard, theelectrolyte employed during the first stage may contain an acidiccompound, such as hydrochloric acid, nitric acid, sulfuric acid,phosphoric acid, polyphosphoric acid, boric acid, boronic acid, etc.Such an electrolyte may have an electrical conductivity of from about0.1 to about 100 mS/cm, in some embodiments from about 0.2 to about 20mS/cm, and in some embodiments, from about 1 to about 10 mS/cm,determined at a temperature of 25° C. The electrolyte employed duringthe second stage typically contains a salt of a weak acid so that thehydronium ion concentration increases in the pores as a result of chargepassage therein. Ion transport or diffusion is such that the weak acidanion moves into the pores as necessary to balance the electricalcharges. As a result, the concentration of the principal conductingspecies (hydronium ion) is reduced in the establishment of equilibriumbetween the hydronium ion, acid anion, and undissociated acid, thusforms a poorer-conducting species. The reduction in the concentration ofthe conducting species results in a relatively high voltage drop in theelectrolyte, which hinders further anodization in the interior while athicker oxide layer is being built up on the outside to a higherformation voltage in the region of continued high conductivity. Suitableweak acid salts may include, for instance, ammonium or alkali metalsalts (e.g., sodium, potassium, etc.) of boric acid, boronic acid,acetic acid, oxalic acid, lactic acid, adipic acid, etc. Particularlysuitable salts include sodium tetraborate and ammonium pentaborate. Suchelectrolytes typically have an electrical conductivity of from about 0.1to about 20 mS/cm, in some embodiments from about 0.5 to about 10 mS/cm,and in some embodiments, from about 1 to about 5 mS/cm, determined at atemperature of 25° C.

If desired, each stage of anodization may be repeated for one or morecycles to achieve the desired dielectric thickness. Furthermore, theanode may also be rinsed or washed with another solvent (e.g., water)after the first and/or second stages to remove the electrolyte.

III. Solid Electrolyte

A solid electrolyte overlies the dielectric that generally functions asthe cathode for the capacitor. The solid electrolyte contains aconductive polymer, which is typically π-conjugated and has electricalconductivity after oxidation or reduction, such as an electricalconductivity of at least about 1 μS/cm. Examples of such π-conjugatedconductive polymers include, for instance, polyheterocycles (e.g.,polypyrroles, polythiophenes, polyanilines, etc.), polyacetylenes,poly-p-phenylenes, polyphenolates, and so forth. In one embodiment, forexample, the polymer is a substituted polythiophene, such as thosehaving the following general structure:

wherein,

T is O or S;

D is an optionally substituted C₁ to C₅ alkylene radical (e.g.,methylene, ethylene, n-propylene, n-butylene, n-pentylene, etc.);

R₇ is a linear or branched, optionally substituted C₁ to C₁₈ alkylradical (e.g., methyl, ethyl, n- or iso-propyl, n-, iso-, sec- ortert-butyl, n-pentyl, 1-methylbutyl, 2-methylbutyl, 3-methylbutyl,1-ethylpropyl, 1,1-dimethylpropyl, 1,2-dimethylpropyl,2,2-dimethylpropyl, n-hexyl, n-heptyl, n-octyl, 2-ethylhexyl, n-nonyl,n-decyl, n-undecyl, n-dodecyl, n-tridecyl, n-tetradecyl, n-hexadecyl,n-octadecyl, etc.); optionally substituted C₅ to C₁₂ cycloalkyl radical(e.g., cyclopentyl, cyclohexyl, cycloheptyl, cyclooctyl, cyclononyicyclodecyl, etc.); optionally substituted C₆ to C₁₄ aryl radical (e.g.,phenyl, naphthyl, etc.); optionally substituted C₇ to C₁₈ aralkylradical (e.g., benzyl, o-, m-, p-tolyl, 2,3-, 2,4-, 2,5-, 2-6, 3-4-,3,5-xylyl, mesityl, etc.); optionally substituted C₁ to C₄ hydroxyalkylradical, or hydroxyl radical; and

q is an integer from 0 to 8, in some embodiments, from 0 to 2, and inone embodiment, 0; and

n is from 2 to 5,000, in some embodiments from 4 to 2,000, and in someembodiments, from 5 to 1,000. Example of substituents for the radicals“D” or “R₇” include, for instance, alkyl, cycloalkyl, aryl, aralkyl,alkoxy, halogen, ether, thioether, disulphide, sulfoxide, sulfone,sulfonate, amino, aldehyde, keto, carboxylic acid ester, carboxylicacid, carbonate, carboxylate, cyano, alkylsilane and alkoxysilanegroups, carboxylamide groups, and so forth.

Particularly suitable thiophene polymers are those in which “D” is anoptionally substituted C₂ to C₃ alkylene radical. For instance, thepolymer may be optionally substituted poly(3,4-ethylenedioxythiophene),which has the following general structure:

Methods for forming conductive polymers, such as described above, arewell known in the art. For instance, U.S. Pat. No. 6,987,663 to Merker,et al., which is incorporated herein in its entirety by referencethereto for all purposes, describes various techniques for formingsubstituted polythiophenes from a monomeric precursor. The monomericprecursor may, for instance, have the following structure:

wherein,

T, D, R₇, and q are defined above. Particularly suitable thiophenemonomers are those in which “D” is an optionally substituted C₂ to C₃alkylene radical. For instance, optionally substituted3,4-alkylenedioxythiophenes may be employed that have the generalstructure:

wherein, R₇ and q are as defined above. In one particular embodiment,“q” is 0. One commercially suitable example of 3,4-ethylenedioxthiopheneis available from H. C. Starck GmbH under the designation Clevios™ M.Other suitable monomers are also described in U.S. Pat. No. 5,111,327 toBlohm, et al. and U.S. Pat. No. 6,635,729 to Groenendaal, et al., whichare incorporated herein in their entirety by reference thereto for allpurposes. Derivatives of these monomers may also be employed that are,for example, dimers or trimers of the above monomers. Higher molecularderivatives, i.e., tetramers, pentamers, etc. of the monomers aresuitable for use in the present invention. The derivatives may be madeup of identical or different monomer units and used in pure form and ina mixture with one another and/or with the monomers. Oxidized or reducedforms of these precursors may also be employed.

To form an in situ polymerized layer, the monomers are chemicallypolymerized in the presence of an oxidative catalyst. The oxidativecatalyst typically includes a transition metal cation, such asiron(III), copper(II), chromium(VI), cerium(IV), manganese(IV),manganese(VII), or ruthenium(III) cations, and etc. A dopant may also beemployed to provide excess charge to the conductive polymer andstabilize the conductivity of the polymer. The dopant typically includesan inorganic or organic anion, such as an ion of a sulfonic acid. Incertain embodiments, the oxidative catalyst has both a catalytic anddoping functionality in that it includes a cation (e.g., transitionmetal) and an anion (e.g., sulfonic acid). For example, the oxidativecatalyst may be a transition metal salt that includes iron(III) cations,such as iron(III) halides (e.g., FeCl₃) or iron(III) salts of otherinorganic acids, such as Fe(ClO₄)₃ or Fe₂(SO₄)₃ and the iron(III) saltsof organic acids and inorganic acids comprising organic radicals.Examples of iron (III) salts of inorganic acids with organic radicalsinclude, for instance, iron(III) salts of sulfuric acid monoesters of C₁to C₂₀ alkanols (e.g., iron(III) salt of lauryl sulfate). Likewise,examples of iron(III) salts of organic acids include, for instance,iron(III) salts of C₁ to C₂₀ alkane sulfonic acids (e.g., methane,ethane, propane, butane, or dodecane sulfonic acid); iron (III) salts ofaliphatic perfluorosulfonic acids (e.g., trifluoromethane sulfonic acid,perfluorobutane sulfonic acid, or perfluorooctane sulfonic acid); iron(III) salts of aliphatic C₁ to C₂₀ carboxylic acids (e.g.,2-ethylhexylcarboxylic acid); iron (III) salts of aliphaticperfluorocarboxylic acids (e.g., trifluoroacetic acid or perfluorooctaneacid); iron (III) salts of aromatic sulfonic acids optionallysubstituted by C₁ to C₂₀ alkyl groups (e.g., benzene sulfonic acid,o-toluene sulfonic acid, p-toluene sulfonic acid, or dodecylbenzenesulfonic acid); iron (III) salts of cycloalkane sulfonic acids (e.g.,camphor sulfonic acid); and so forth. Mixtures of these above-mentionediron(III) salts may also be used. Iron(III)-p-toluene sulfonate,iron(III)-o-toluene sulfonate, and mixtures thereof, are particularlysuitable. One commercially suitable example of iron(III)-p-toluenesulfonate is available from Heraeus Clevios under the designationClevios™ C.

The oxidative catalyst and monomer may be applied either sequentially ortogether to initiate the polymerization reaction. Suitable applicationtechniques for applying these components include screen-printing,dipping, electrophoretic coating, and spraying. As an example, themonomer may initially be mixed with the oxidative catalyst to form aprecursor solution. Once the mixture is formed, it may be applied to theanode part and then allowed to polymerize so that a conductive coatingis formed on the surface. Alternatively, the oxidative catalyst andmonomer may be applied sequentially. In one embodiment, for example, theoxidative catalyst is dissolved in an organic solvent (e.g., butanol)and then applied as a dipping solution. The anode part may then be driedto remove the solvent therefrom. Thereafter, the part may be dipped intoa solution containing the monomer. Regardless, polymerization istypically performed at temperatures of from about −10° C. to about 250°C., and in some embodiments, from about 0° C. to about 200° C.,depending on the oxidizing agent used and desired reaction time.Suitable polymerization techniques, such as described above, may bedescribed in more detail in U.S. Pat. No. 7,515,396 to Biler. Stillother methods for applying such conductive coating(s) may be describedin U.S. Pat. No. 5,457,862 to Sakata, et al., U.S. Pat. No. 5,473,503 toSakata, et al., U.S. Pat. No. 5,729,428 to Sakata, et al., and U.S. Pat.No. 5,812,367 to Kudoh, et al., which are incorporated herein in theirentirety by reference thereto for all purposes.

In addition to an in situ polymerized conductive polymer, the solidelectrolyte also contains a hydroxy-functional nonionic polymer. Theterm “hydroxy-functional” generally means that the compound contains atleast one hydroxyl functional group or is capable of possessing such afunctional group in the presence of a solvent. Without intending to belimited by theory, it is believed that hydroxy-functional nonionicpolymers can improve the degree of contact between the polymer particlesand the surface of the internal dielectric, which is typicallyrelatively smooth in nature as a result of higher forming voltages. Thisunexpectedly increases the capacitance of the resulting capacitor at lowtemperatures. Furthermore, it is believed that the use of ahydroxy-functional polymer with a certain molecular weight can alsominimize the likelihood of chemical decomposition. For instance, themolecular weight of the hydroxy-functional polymer may be from about 100to 10,000 grams per mole, in some embodiments from about 200 to 2,000,in some embodiments from about 300 to about 1,200, and in someembodiments, from about 400 to about 800.

Any of a variety of hydroxy-functional nonionic polymers may generallybe employed for this purpose. In one embodiment, for example, thehydroxy-functional polymer is a polyalkylene ether. Polyalkylene ethersmay include polyalkylene glycols (e.g., polyethylene glycols,polypropylene glycols polytetramethylene glycols, polyepichlorohydrins,etc.), polyoxetanes, polyphenylene ethers, polyether ketones, and soforth. Polyalkylene ethers are typically predominantly linear, nonionicpolymers with terminal hydroxy groups. Particularly suitable arepolyethylene glycols, polypropylene glycols and polytetramethyleneglycols (polytetrahydrofurans), which are produced by polyaddition ofethylene oxide, propylene oxide or tetrahydrofuran onto water. Thepolyalkylene ethers may be prepared by polycondensation reactions fromdiols or polyols. The dial component may be selected, in particular,from saturated or unsaturated, branched or unbranched, aliphaticdihydroxy compounds containing 5 to 36 carbon atoms or aromaticdihydroxy compounds, such as, for example, pentane-1,5-diol,hexane-1,6-diol, neopentyl glycol, bis-(hydroxymethyl)-cyclohexanes,bisphenol A, dimer diols, hydrogenated dimer diols or even mixtures ofthe diols mentioned. In addition, polyhydric alcohols may also be usedin the polymerization reaction, including for example glycerol, di- andpolyglycerol, trimethylolpropane, pentaerythritol or sorbitol.

In addition to those noted above, other hydroxy-functional nonionicpolymers may also be employed in the present invention. Some examples ofsuch polymers include, for instance, ethoxylated alkylphenols;ethoxylated or propoxylated C₆-C₂₄ fatty alcohols; polyoxyethyleneglycol alkyl ethers having the general formula:CH₃—(CH₂)₁₀₋₁₆—(O—C₂H₄)₁₋₂₅—OH (e.g., octaethylene glycol monododecylether and pentaethylene glycol monododecyl ether); polyoxypropyleneglycol alkyl ethers having the general formula:CH₃—(CH₂)₁₀₋₁₆—(O—C₃H₆)₁₋₂₅—OH; polyoxyethylene glycol octylphenolethers having the following general formula:C₈H₁₇—(C₆H₄)—(O—C₂H₄)₁₋₂₅—OH (e.g., Triton™ X-100); polyoxyethyleneglycol alkylphenol ethers having the following general formula:C₉H₁₉—(C₆H₄)—(O—C₂H₄)₁₋₂₅—OH (e.g., nonoxynol-9); polyoxyethylene glycolesters of C₈-C₂₄ fatty acids, such as polyoxyethylene glycol sorbitanalkyl esters (e.g., polyoxyethylene (20) sorbitan monolaurate,polyoxyethylene (20) sorbitan monopalmitate, polyoxyethylene (20)sorbitan monostearate, polyoxyethylene (20) sorbitan monooleate, PEG-20methyl glucose distearate, PEG-20 methyl glucose sesquistearate, PEG-80castor oil, and PEG-20 castor oil, PEG-3 castor oil, PEG 600 dioleate,and PEG 400 dioleate) and polyoxyethylene glycerol alkyl esters (e.g.,polyoxyethylene-23 glycerol laurate and polyoxyethylene-20 glycerolstearate); polyoxyethylene glycol ethers of C₈-C₂₄ fatty acids (e.g.,polyoxyethylene-10 cetyl ether, polyoxyethylene-10 stearyl ether,polyoxyethylene-20 cetyl ether, polyoxyethylene-10 oleyl ether,polyoxyethylene-20 oleyl ether, polyoxyethylene-20 isohexadecyl ether,polyoxyethylene-15 tridecyl ether, and polyoxyethylene-6 tridecylether); block copolymers of polyethylene glycol and polypropylene glycol(e.g., Poloxamers); and so forth, as well as mixtures thereof.

The hydroxy-functional nonionic polymer may be incorporated into thesolid electrolyte in a variety of different ways. In certainembodiments, for instance, the hydroxy-functional polymer may simply beincorporated into any layer(s) formed by the in situ polymerizationprocess described above. In such embodiments, the concentration of thehydroxy-functional polymer in the polymerization solution may be fromabout 1 wt. % to about 50 wt. %, in some embodiments from about 5 wt. %to about 40 wt. %, and in some embodiments, from about 10 wt. % to about30 wt. %.

In other embodiments, however, the hydroxy-functional polymer may beapplied after the initial polymer layer(s) are formed. In suchembodiments, the technique used to apply the hydroxy-functional polymermay vary. For example, the polymer may be applied in the form of aliquid solution using various methods, such as immersion, dipping,pouring, dripping, injection, spraying, spreading, painting or printing,for example, inkjet, screen printing or tampon printing. Solvents knownto the person skilled in the art can be employed in the solution, suchas water, alcohols, or a mixture thereof. The concentration of thehydroxy-functional polymer in such a solution typically ranges fromabout 5 wt. % to about 95 wt. %, in some embodiments from about 10 wt. %to about 70 wt. %, and in some embodiments, from about 15 wt. % to about50 wt. % of the solution. If desired, such solutions may be generallyfree of conductive polymers. For example, conductive polymers mayconstitute about 2 wt. % or less, in some embodiments about 1 wt. % orless, and in some embodiments, about 0.5 wt. % or less of the solution.

Alternatively, however, it may also be desired to employ a conductivepolymer in combination with the hydroxy-functional polymer. For example,in certain embodiments, a polymer dispersion that contains conductivepolymer particles and a hydroxy-functional polymer may be applied to theanode after the initial layer(s) formed from in situ polymerization areapplied. One benefit to employing such a dispersion is that it may beable to increase the adhesion and coverage of the in situ polymerizedlayer(s). In this regard, the particles employed in the dispersiontypically have a small size, such as an average diameter of from about 1to about 200 nanometers, in some embodiments from about 2 to about 150nanometers, and in some embodiments, from about 5 to about 50nanometers. The diameter of the particles may be determined using knowntechniques, such as by ultracentrifuge, laser diffraction, etc. Theshape of the particles may likewise vary. In one particular embodiment,for instance, the particles are spherical in shape. However, it shouldbe understood that other shapes are also contemplated by the presentinvention, such as plates, rods, discs, bars, tubes, irregular shapes,etc. The concentration of the particles in the dispersion may varydepending on the desired viscosity of the dispersion and the particularmanner in which the dispersion is to be applied to the capacitor.Typically, however, the particles constitute from about 0.1 to about 10wt. %, in some embodiments from about 0.4 to about 5 wt. %, and in someembodiments, from about 0.5 to about 4 wt. % of the dispersion.

The dispersion also generally contains a counterion that enhances thestability of the particles. That is, the conductive polymer (e.g.,polythiophene or derivative thereof) typically has a charge on the mainpolymer chain that is neutral or positive (cationic). Polythiophenederivatives, for instance, typically carry a positive charge in the mainpolymer chain. In some cases, the polymer may possess positive andnegative charges in the structural unit, with the positive charge beinglocated on the main chain and the negative charge optionally on thesubstituents of the radical “R”, such as sulfonate or carboxylategroups. The positive charges of the main chain may be partially orwholly saturated with the optionally present anionic groups on theradicals “R.” Viewed overall, the polythiophenes may, in these cases, becationic, neutral or even anionic. Nevertheless, they are all regardedas cationic polythiophenes as the polythiophene main chain has apositive charge.

The counterion may be a monomeric or polymeric anion that counteractsthe charge of the conductive polymer. Polymeric anions can, for example,be anions of polymeric carboxylic acids (e.g., polyacrylic acids,polymethacrylic acid, polymaleic acids, etc.); polymeric sulfonic acids(e.g., polystyrene sulfonic acids (“PSS”), polyvinyl sulfonic acids,etc.); and so forth. The acids may also be copolymers, such ascopolymers of vinyl carboxylic and vinyl sulfonic acids with otherpolymerizable monomers, such as acrylic acid esters and styrene.Likewise, suitable monomeric anions include, for example, anions of C₁to C₂₀ alkane sulfonic acids (e.g., dodecane sulfonic acid); aliphaticperfluorosulfonic acids (e.g., trifluoromethane sulfonic acid,perfluorobutane sulfonic acid or perfluorooctane sulfonic acid);aliphatic C₁ to C₂₀ carboxylic acids (e.g., 2-ethyl-hexylcarboxylicacid); aliphatic perfluorocarboxylic acids (e.g., trifluoroacetic acidor perfluorooctanoic acid); aromatic sulfonic acids optionallysubstituted by C₁ to C₂₀ alkyl groups (e.g., benzene sulfonic acid,o-toluene sulfonic acid, p-toluene sulfonic acid or dodecylbenzenesulfonic acid); cycloalkane sulfonic acids (e.g., camphor sulfonic acidor tetrafluoroborates, hexafluorophosphates, perchiorates,hexafluoroantimonates, hexafluoroarsenates or hexachloroantimonates);and so forth. Particularly suitable counteranions are polymeric anions,such as a polymeric carboxylic or sulfonic acid (e.g., polystyrenesulfonic acid (“PSS”)). The molecular weight of such polymeric anionstypically ranges from about 1,000 to about 2,000,000, and in someembodiments, from about 2,000 to about 500,000.

When employed, the weight ratio of such counterions to conductivepolymers in the dispersion and in the resulting layer is typically fromabout 0.5:1 to about 50:1, in some embodiments from about 1:1 to about30:1, and in some embodiments, from about 2:1 to about 20:1. The weightof the electrically conductive polymers corresponds referred to theabove-referenced weight ratios refers to the weighed-in portion of themonomers used, assuming that a complete conversion occurs duringpolymerization.

In addition to conductive polymer(s) and counterion(s), the dispersionmay also contain one or more binders to further enhance the adhesivenature of the polymeric layer and also increase the stability of theparticles within the dispersion. The binders may be organic in nature,such as polyvinyl alcohols, polyvinyl pyrrolidones, polyvinyl chlorides,polyvinyl acetates, polyvinyl butyrates, polyacrylic acid esters,polyacrylic acid amides, polymethacrylic acid esters, polymethacrylicacid amides, polyacrylonitriles, styrene/acrylic acid ester, vinylacetate/acrylic acid ester and ethylene/vinyl acetate copolymers,polybutadienes, polyisoprenes, polystyrenes, polyethers, polyesters,polycarbonates, polyurethanes, polyamides, polyimides, polysulfones,melamine formaldehyde resins, epoxide resins, silicone resins orcelluloses. Crosslinking agents may also be employed to enhance theadhesion capacity of the binders. Such crosslinking agents may include,for instance, melamine compounds, masked isocyanates or functionalsilanes, such as 3-glycidoxypropyltrialkoxysilane, tetraethoxysilane andtetraethoxysilane hydrolysate or crosslinkable polymers, such aspolyurethanes, polyacrylates or polyolefins, and subsequentcrosslinking.

Dispersion agents may also be employed to facilitate the formation ofthe solid electrolyte and the ability to apply it to the anode part.Suitable dispersion agents include solvents, such as aliphatic alcohols(e.g., methanol, ethanol, i-propanol and butanol), aliphatic ketones(e.g., acetone and methyl ethyl ketones), aliphatic carboxylic acidesters (e.g., ethyl acetate and butyl acetate), aromatic hydrocarbons(e.g., toluene and xylene), aliphatic hydrocarbons (e.g., hexane,heptane and cyclohexane), chlorinated hydrocarbons (e.g.,dichloromethane and dichloroethane), aliphatic nitriles (e.g.,acetonitrile), aliphatic sulfoxides and sulfones (e.g., dimethylsulfoxide and sulfolane), aliphatic carboxylic acid amides (e.g.,methylacetamide, dimethylacetamide and dimethylformamide), aliphatic andaraliphatic ethers (e.g., diethylether and anisole), water, and mixturesof any of the foregoing solvents. A particularly suitable dispersionagent is water.

In addition to those mentioned above, still other ingredients may alsobe used in the dispersion. For example, conventional fillers may be usedthat have a size of from about 10 nanometers to about 100 micrometers,in some embodiments from about 50 nanometers to about 50 micrometers,and in some embodiments, from about 100 nanometers to about 30micrometers. Examples of such fillers include calcium carbonate,silicates, silica, calcium or barium sulfate, aluminum hydroxide, glassfibers or bulbs, wood flour, cellulose powder carbon black, electricallyconductive polymers, etc. The fillers may be introduced into thedispersion in powder form, but may also be present in another form, suchas fibers.

Surface-active substances may also be employed in the dispersion, suchas ionic or non-ionic surfactants. Furthermore, adhesives may beemployed, such as organofunctional silanes or their hydrolysates, forexample 3-glycidoxypropyltrialkoxysilane, 3-aminopropyl-triethoxysilane,3-mercaptopropyl-trimethoxysilane, 3-metacryloxypropyltrimethoxysilane,vinyltrimethoxysilane or octyltriethoxysilane. The dispersion may alsocontain additives that increase conductivity, such as ethergroup-containing compounds (e.g., tetrahydrofuran), lactonegroup-containing compounds (e.g., γ-butyrolactone or γ-valerolactone),amide or lactam group-containing compounds (e.g., caprolactam,N-methylcaprolactam, N,N-dimethylacetamide, N-methylacetamide,N,N-dimethylformamide (DMF), N-methylformamide, N-methylformanilide,N-methylpyrrolidone (NMP), N-octylpyrrolidone, or pyrrolidone), sulfonesand sulfoxides (e.g., sulfolane (tetramethylenesulfone) ordimethylsulfoxide (DMSO)), sugar or sugar derivatives (e.g., saccharose,glucose, fructose, or lactose), sugar alcohols (e.g., sorbitol ormannitol), furan derivatives (e.g., 2-furancarboxylic acid or3-furancarboxylic acid), an alcohols (e.g., ethylene glycol, glycerol,di- or triethylene glycol).

The polymeric dispersion may be applied by to the part using a varietyof known techniques, such as by spin coating, impregnation, pouring,dropwise application, injection, spraying, doctor blading, brushing,printing (e.g., ink-jet, screen, or pad printing), or dipping. Althoughit may vary depending on the application technique employed, theviscosity of the dispersion is typically from about 0.1 to about 100,000mPas (measured at a shear rate of 100 s⁻¹), in some embodiments fromabout 1 to about 10,000 mPas, in some embodiments from about 10 to about1,500 mPas, and in some embodiments, from about 100 to about 1000 mPas.Once applied, the layer may be dried and/or washed. One or moreadditional layers may also be formed in this manner to achieve thedesired thickness.

Regardless of the particular manner in which it is formed, theconcentration of the hydroxy-functional polymer in the polymerdispersion is typically from about 1 wt. % to about 50 wt. %, in someembodiments from about 5 wt. % to about 40 wt. %, and in someembodiments, from about 10 wt. % to about 30 wt. %. Likewise, in thoseembodiments in which the hydroxy-functional polymer is employed in apolymer dispersion, it may also be desirable that the in situpolymerized layer(s) are generally free of such hydroxy-functionalnonionic polymers. For example, hydroxy-functional polymers mayconstitute about 2 wt. % or less, in some embodiments about 1 wt. % orless, and in some embodiments, about 0.5 wt. % or less of the in situpolymerized layer(s). Once applied, the layer formed by the polymerdispersion may be dried and/or washed. One or more additional layers mayalso be formed in this manner to achieve the desired thickness.Typically, the total thickness of the layers formed by the polymerdispersion is from about 1 to about 50 μm, and in some embodiments, fromabout 5 to about 20 μm.

IV. External Polymer Coating

Although not required, an external polymer coating may also be appliedto the anode body and overlie the solid electrolyte. The externalpolymer coating generally contains one or more layers formed from adispersion of pre-polymerized conductive particles, such as described inmore detail above. The external coating may be able to further penetrateinto the edge region of the capacitor body to increase the adhesion tothe dielectric and result in a more mechanically robust part, which mayreduce equivalent series resistance and leakage current. Because it isgenerally intended to improve the degree of edge coverage rather toimpregnate the interior of the anode, the particles used in the externalcoating typically have a larger size than those employed in any optionaldispersions of the solid electrolyte. For example, the ratio of theaverage size of the particles employed in the external polymer coatingto the average size of the particles employed in any dispersion of thesolid electrolyte is typically from about 1.5 to about 30, in someembodiments from about 2 to about 20, and in some embodiments, fromabout 5 to about 15. For example, the particles employed in thedispersion of the external coating may have an average size of fromabout 50 to about 500 nanometers, in some embodiments from about 80 toabout 250 nanometers, and in some embodiments, from about 100 to about200 nanometers.

If desired, a crosslinking agent may also be employed in the externalpolymer coating to enhance the degree of adhesion to the solidelectrolyte. Typically, the crosslinking agent is applied prior toapplication of the dispersion used in the external coating. Suitablecrosslinking agents are described, for instance, in U.S. PatentPublication No. 2007/0064376 to Merker, et al. and include, forinstance, amines (e.g., diamines, triamines, oligomer amines,polyamines, etc.); polyvalent metal cations, such as salts or compoundsof Mg, Al, Ca, Fe, Cr, Mn, Ba, Ti, Co, Ni, Cu, Ru, Ce or Zn, phosphoniumcompounds, sulfonium compounds, etc. Particularly suitable examplesinclude, for instance, 1,4-diaminocyclohexane,1,4-bis(amino-methyl)cyclohexane, ethylenediamine, 1,6-hexanediamine,1,7-heptanediamine, 1,8-octanediamine, 1,9-nonanediamine,1,10-decanediamine, 1,12-dodecanediamine, N,N-dimethylethylenediamine,N,N,N′,N′-tetramethylethylenediamine,N,N,N′,N′-tetramethyl-1,4-butanediamine, etc., as well as mixturesthereof.

The crosslinking agent is typically applied from a solution ordispersion whose pH is from 1 to 10, in some embodiments from 2 to 7, insome embodiments, from 3 to 6, as determined at 25° C. Acidic compoundsmay be employed to help achieve the desired pH level. Examples ofsolvents or dispersants for the crosslinking agent include water ororganic solvents, such as alcohols, ketones, carboxylic esters, etc. Thecrosslinking agent may be applied to the capacitor body by any knownprocess, such as spin-coating, impregnation, casting, dropwiseapplication, spray application, vapor deposition, sputtering,sublimation, knife-coating, painting or printing, for example inkjet,screen or pad printing. Once applied, the crosslinking agent may bedried prior to application of the polymer dispersion. This process maythen be repeated until the desired thickness is achieved. For example,the total thickness of the entire external polymer coating, includingthe crosslinking agent and dispersion layers, may range from about 1 toabout 50 μm, in some embodiments from about 2 to about 40 μm, and insome embodiments, from about 5 to about 20 μm.

V. Other Components of the Capacitor

If desired, the capacitor may also contain other layers as is known inthe art. For example, a protective coating may optionally be formedbetween the dielectric and solid electrolyte, such as one made of arelatively insulative resinous material (natural or synthetic). Suchmaterials may have a specific resistivity of greater than about 10 Ω·cm,in some embodiments greater than about 100, in some embodiments greaterthan about 1,000 Ω·cm, in some embodiments greater than about 1×10⁵Ω·cm, and in some embodiments, greater than about 1×10¹⁰ Ω·cm. Someresinous materials that may be utilized in the present inventioninclude, but are not limited to, polyurethane, polystyrene, esters ofunsaturated or saturated fatty acids (e.g., glycerides), and so forth.For instance, suitable esters of fatty acids include, but are notlimited to, esters of lauric acid, myristic acid, palmitic acid, stearicacid, eleostearic acid, oleic acid, linoleic acid, linolenic acid,aleuritic acid, shellolic acid, and so forth. These esters of fattyacids have been found particularly useful when used in relativelycomplex combinations to form a “drying oil”, which allows the resultingfilm to rapidly polymerize into a stable layer. Such drying oils mayinclude mono-, di-, and/or tri-glycerides, which have a glycerolbackbone with one, two, and three, respectively, fatty acyl residuesthat are esterified. For instance, some suitable drying oils that may beused include, but are not limited to, olive oil, linseed oil, castoroil, tung oil, soybean oil, and shellac. These and other protectivecoating materials are described in more detail U.S. Pat. No. 6,674,635to Fife, et al., which is incorporated herein in its entirety byreference thereto for all purposes.

If desired, the part may also be applied with a carbon layer (e.g.,graphite) and silver layer, respectively. The silver coating may, forinstance, act as a solderable conductor, contact layer, and/or chargecollector for the capacitor and the carbon coating may limit contact ofthe silver coating with the solid electrolyte. Such coatings may coversome or all of the solid electrolyte.

The capacitor may also be provided with terminations, particularly whenemployed in surface mounting applications. For example, the capacitormay contain an anode termination to which the anode lead of thecapacitor element is electrically connected and a cathode termination towhich the cathode of the capacitor element is electrically connected.Any conductive material may be employed to form the terminations, suchas a conductive metal (e.g., copper, nickel, silver, nickel, zinc, tin,palladium, lead, copper, aluminum, molybdenum, titanium, iron,zirconium, magnesium, and alloys thereof). Particularly suitableconductive metals include, for instance, copper, copper alloys (e.g.,copper-zirconium, copper-magnesium, copper-zinc, or copper-iron),nickel, and nickel alloys (e.g., nickel-iron). The thickness of theterminations is generally selected to minimize the thickness of thecapacitor. For instance, the thickness of the terminations may rangefrom about 0.05 to about 1 millimeter, in some embodiments from about0.05 to about 0.5 millimeters, and from about 0.07 to about 0.2millimeters. One exemplary conductive material is a copper-iron alloymetal plate available from Wieland (Germany). If desired, the surface ofthe terminations may be electroplated with nickel, silver, gold, tin,etc. as is known in the art to ensure that the final part is mountableto the circuit board. In one particular embodiment, both surfaces of theterminations are plated with nickel and silver flashes, respectively,while the mounting surface is also plated with a tin solder layer.

Referring to FIG. 1, one embodiment of an electrolytic capacitor 30 isshown that includes an anode termination 62 and a cathode termination 72in electrical connection with a capacitor element 33. The capacitorelement 33 has an upper surface 37, lower surface 39, front surface 36,and rear surface 38. Although it may be in electrical contact with anyof the surfaces of the capacitor element 33, the cathode termination 72in the illustrated embodiment is in electrical contact with the lowersurface 39 and rear surface 38. More specifically, the cathodetermination 72 contains a first component 73 positioned substantiallyperpendicular to a second component 74. The first component 73 is inelectrical contact and generally parallel with the lower surface 39 ofthe capacitor element 33. The second component 74 is in electricalcontact and generally parallel to the rear surface 38 of the capacitorelement 33. Although depicted as being integral, it should be understoodthat these portions may alternatively be separate pieces that areconnected together, either directly or via an additional conductiveelement (e.g., metal).

The anode termination 62 likewise contains a first component 63positioned substantially perpendicular to a second component 64. Thefirst component 63 is in electrical contact and generally parallel withthe lower surface 39 of the capacitor element 33. The second component64 contains a region 51 that carries an anode lead 16. In theillustrated embodiment, the region 51 possesses a “U-shape” for furtherenhancing surface contact and mechanical stability of the lead 16.

The terminations may be connected to the capacitor element using anytechnique known in the art. In one embodiment, for example, a lead framemay be provided that defines the cathode termination 72 and anodetermination 62. To attach the electrolytic capacitor element 33 to thelead frame, a conductive adhesive may initially be applied to a surfaceof the cathode termination 72. The conductive adhesive may include, forinstance, conductive metal particles contained with a resin composition.The metal particles may be silver, copper, gold, platinum, nickel, zinc,bismuth, etc. The resin composition may include a thermoset resin (e.g.,epoxy resin), curing agent (e.g., acid anhydride), and coupling agent(e.g., silane coupling agents). Suitable conductive adhesives may bedescribed in U.S. Patent Application Publication No. 2006/0038304 toOsako, et al., which is incorporated herein in its entirety by referencethereto for all purposes. Any of a variety of techniques may be used toapply the conductive adhesive to the cathode termination 72. Printingtechniques, for instance, may be employed due to their practical andcost-saving benefits.

A variety of methods may generally be employed to attach theterminations to the capacitor. In one embodiment, for example, thesecond component 64 of the anode termination 62 and the second component74 of the cathode termination 72 are initially bent upward to theposition shown in FIG. 1. Thereafter, the capacitor element 33 ispositioned on the cathode termination 72 so that its lower surface 39contacts the adhesive and the anode lead 16 is received by the upperU-shaped region 51. If desired, an insulating material (not shown), suchas a plastic pad or tape, may be positioned between the lower surface 39of the capacitor element 33 and the first component 63 of the anodetermination 62 to electrically isolate the anode and cathodeterminations.

The anode lead 16 is then electrically connected to the region 51 usingany technique known in the art, such as mechanical welding, laserwelding, conductive adhesives, etc. For example, the anode lead 16 maybe welded to the anode termination 62 using a laser. Lasers generallycontain resonators that include a laser medium capable of releasingphotons by stimulated emission and an energy source that excites theelements of the laser medium. One type of suitable laser is one in whichthe laser medium consist of an aluminum and yttrium garnet (YAG), dopedwith neodymium (Nd). The excited particles are neodymium ions Nd³⁺. Theenergy source may provide continuous energy to the laser medium to emita continuous laser beam or energy discharges to emit a pulsed laserbeam. Upon electrically connecting the anode lead 16 to the anodetermination 62, the conductive adhesive may then be cured. For example,a heat press may be used to apply heat and pressure to ensure that theelectrolytic capacitor element 33 is adequately adhered to the cathodetermination 72 by the adhesive.

Once the capacitor element is attached, the lead frame is enclosedwithin a resin casing, which may then be filled with silica or any otherknown encapsulating material. The width and length of the case may varydepending on the intended application. Suitable casings may include, forinstance, “A”, “B”, “C”, “D”, “E”, “F”, “G”, “H”, “J”, “K”, “L”, “M”,“N”, “P”, “R”, “S”, “T”, “V”, “W”, “Y”, “X”, or “Z” (AVX Corporation).Regardless of the case size employed, the capacitor element isencapsulated so that at least a portion of the anode and cathodeterminations are exposed for mounting onto a circuit board. As shown inFIG. 1, for instance, the capacitor element 33 is encapsulated in a case28 so that a portion of the anode termination 62 and a portion of thecathode termination 72 are exposed.

As a result of the present invention, the capacitor assembly may exhibitexcellent electrical properties. For example, the leakage current, whichgenerally refers to the current flowing from one conductor to anadjacent conductor through an insulator, can be maintained at relativelylow levels. For example, the numerical value of the normalized leakagecurrent of a capacitor of the present invention is, in some embodiments,less than about 1 μA/μF*V, in some embodiments less than about 0.5μA/μF*V, and in some embodiments, less than about 0.1 μA/μF*V, where μAis microamps and uF*V is the product of the capacitance and the ratedvoltage. Such normalized leakage current values may even be maintainedafter aging for a substantial amount of time at high temperatures. Forexample, the values may be maintained for about 100 hours or more, insome embodiments from about 300 hours to about 3000 hours, and in someembodiments, from about 400 hours to about 2500 hours (e.g., 500 hours,600 hours, 700 hours, 800 hours, 900 hours, 1000 hours, 1100 hours, 1200hours, or 2000 hours) at temperatures ranging from about −55° C. toabout 250° C., in some embodiments from about 0° C. to about 225° C.,and in some embodiments, from about 10° C. to about 225° C.

The present invention may be better understood by reference to thefollowing examples.

Test Procedures

Equivalent Series Resistance (ESR)

Equivalence series resistance may be measured using a Keithley 3330Precision LCZ meter with Kelvin Leads 2.2 volt DC bias and a 0.5 voltpeak to peak sinusoidal signal. The operating frequency was 100 kHz andthe temperature was room temperature.

Dry and Wet Capacitance

The capacitance was measured using a Keithley 3330 Precision LCZ meterwith Kelvin Leads with 2.2 volt DC bias and a 0.5 volt peak to peaksinusoidal signal. The operating frequency was 120 Hz and thetemperature was room temperature. The “dry capacitance” refers to thecapacitance of the part after application of the solid electrolyte,graphite, and silver layers, while the “wet capacitance” refers to thecapacitance of the part after formation of the dielectric, measured in17% sulfuric acid in reference to 1 mF tantalum cathode.

Temperature Stability Characteristics:

Temperature stability was conducted at the temperature of 125° C.without voltage. The ESR and capacitance of an individual capacitor wererecorded after 3, 6, 12 and 18 days.

EXAMPLE 1

70,000 μFV/g tantalum powder was used to form anode samples. Each anodesample was embedded with a tantalum wire, sintered at 1280° C., andpressed to a density of 6.8 g/cm3. The resulting pellets had a size of1.80×1.20×2.40 mm. The pellets were anodized to 14.4V inwater/phosphoric acid electrolyte with conductivity 8.6 mS attemperature of 85° C. to form the dielectric layer. A conductive polymercoating was then formed by dipping the anodes into a butanol solution ofiron (III) toluenesulfonate (Clevios™ C, H. C. Starck) for 5 minutes andconsequently into 3,4-ethylenedioxythiophene (Clevios™ M, H. C. Starck)for 1 minute. After 45 minutes of polymerization, a thin layer ofpoly(3,4-ethylenedioxythiophene) was formed on the surface of thedielectric. The parts were washed in methanol to remove reactionby-products, anodized in a liquid electrolyte, and washed again inmethanol. The polymerization cycle was repeated 12 times. The parts werethen dipped into a graphite dispersion and dried. Finally, the partswere dipped into a silver dispersion and dried. Multiple parts (200) of150 μF/6.3V capacitors were made in this manner.

EXAMPLE 2

Capacitors were formed in the manner described in Example 1, except thatusing a different conductive polymer coating. A conductive polymercoating was then formed by dipping the anodes into a butanol solution ofiron (III) toluenesulfonate (Clevios™ C, H. C. Starck) for 5 minutes andconsequently into 3,4-ethylenedioxythiophene (Clevios™ M, H. C. Starck)for 1 minute. After 45 minutes of polymerization, a thin layer ofpoly(3,4-ethylenedioxythiophene) was formed on the surface of thedielectric. The parts were washed in methanol to remove reactionby-products, anodized in a liquid electrolyte, and washed again inmethanol. This process was repeated 6 times. Thereafter, the parts weredipped into a dispersed poly(3,4-ethylenedioxythiophene) having a solidscontent 2% and viscosity 20 mPa·s (Clevios™ K, H. C. Starck). Uponcoating, the parts were dried at 125° C. for 20 minutes. This processwas not repeated. Thereafter, the parts were dipped into a dispersedpoly(3,4-ethylenedioxythiophene) having a solids content 2% andviscosity 160 mPa·s (Clevios™ K, H. C. Starck). Upon coating, the partswere dried at 125° C. for 20 minutes. This process was repeated 8 times.The parts were then dipped into a graphite dispersion and dried.Finally, the parts were dipped into a silver dispersion and dried.Multiple parts (200) of 150 μF/6.3V capacitors were made in this manner.

EXAMPLE 3

Capacitors were formed in the manner described in Example 1, except thatusing a different conductive polymer coating. A conductive polymercoating was then formed by dipping the anodes into a butanol solution ofiron (III) toluenesulfonate (Clevios™ C, H. C. Starck) for 5 minutes andconsequently into 3,4-ethylenedioxythiophene (Clevios™ M, H. C. Starck)for 1 minute. After 45 minutes of polymerization, a thin layer ofpoly(3,4-ethylenedioxythiophene) was formed on the surface of thedielectric. The parts were washed in methanol to remove reactionby-products, anodized in a liquid electrolyte, and washed again inmethanol. This process was repeated 6 times. Thereafter, the parts weredipped into a dispersed poly(3,4-ethylenedioxythiophene) having a solidscontent 2% and viscosity 20 mPa·s (Clevios™ K, H. C. Starck) andadditional 20% solids content of poly(ethylene glycol) with molecularweight 600 (Sigma Aldrich^(R)). Upon coating, the parts were dried at125° C. for 20 minutes. This process was not repeated. Thereafter, theparts were dipped into a dispersed poly(3,4-ethylenedioxythiophene)having a solids content 2% and viscosity 160 mPa·s (Clevios™ K, H. C.Starck). Upon coating, the parts were dried at 125° C. for 20 minutes.This process was repeated 8 times. The parts were then dipped into agraphite dispersion and dried, Finally, the parts were dipped into asilver dispersion and dried. Multiple parts (200) of 150 μF/6.3Vcapacitors were made in this manner.

The finished capacitors of Examples 1-3 were then tested for electricalperformance before an assembly process. The median results of ESR andcapacitance are set forth below in Table 1. The wet capacitance was145.0 μF in all examples.

TABLE 1 Electrical Properties Cap Dry/Wet Cap ESR [μF] [%] [mΩ] Example1 144.7 99.8 65 Example 2 120.2 82.9 44 Example 3 129.2 89.1 45

The finished capacitors of Examples 1-3 were also tested for temperaturestability characteristics without voltage before an assembly process.The median results of ESR and capacitance are set forth below in Table2a,b.

TABLE 2a Temperature Stability Characteristics (ESR[mΩ]) 3 days 6 days12 days 18 days Example 1 69 138 574 N/A Example 2 56 69 110 227 Example3 51 61 83 132

TABLE 2b Temperature Stability Characteristics (Cap[μF]) 3 days 6 days12 days 18 days Example 1 140.3 129.6 39.7 20.7 Example 2 117.3 110.683.6 49.9 Example 3 109.8 104.6 96.0 87.6

As indicated, the parts with polyethylene glycol (Example 3) had ahigher stability under elevated temperature.

These and other modifications and variations of the present inventionmay be practiced by those of ordinary skill in the art, withoutdeparting from the spirit and scope of the present invention. Inaddition, it should be understood that aspects of the variousembodiments may be interchanged both in whole or in part. Furthermore,those of ordinary skill in the art will appreciate that the foregoingdescription is by way of example only, and is not intended to limit theinvention so further described in such appended claims.

What is claimed is:
 1. A solid electrolytic capacitor comprising: asintered porous anode; a dielectric layer that overlies the anode body;and a solid electrolyte overlying the dielectric layer, wherein thesolid electrolyte comprises a first layer that overlies the dielectriclayer and includes an in situ polymerized conductive polymer and asecond layer that overlies the first layer, wherein the second layerincludes a plurality of pre-polymerized conductive polymer particles anda separate hydroxy-functional nonionic polymer.
 2. The solidelectrolytic capacitor of claim 1, wherein the in situ polymerizedconductive polymer and the pre-polymerized conductive polymer particlesinclude is a substituted polythiophene.
 3. The solid electrolyticcapacitor of claim 1, wherein the hydroxy-functional nonionic polymerhas a molecular weight of from about 300 to about 1,200 grams per mole.4. The solid electrolytic capacitor of claim 1, wherein thehydroxy-functional polymer is a polyalkylene ether.
 5. The solidelectrolytic capacitor of claim 4, wherein the polyalkylene ether is apolyalkylene glycol.
 6. The solid electrolytic capacitor of claim 1,wherein the hydroxy-functional polymer is an ethoxylated alkylphenol,ethoxylated or propoxylated C₆-C₂₄ fatty alcohol, polyoxyethylene glycolalkyl ether, polyoxyethylene glycol alkyl phenol ether, polyoxyethyleneglycol ester of a C₈-C₂₄ fatty acid, polyoxyethylene glycol ether of aC₈-C₂₄ fatty acid, block copolymer of polyethylene glycol andpolypropylene glycol, or a combination thereof.
 7. The solidelectrolytic capacitor of claim 2, wherein the substituted polythiopheneis poly(3,4-ethylenedioxythiophene).
 8. The solid electrolytic capacitorof claim 1, wherein the particles contain a monomeric or polymericcounteranion.
 9. The solid electrolytic capacitor of claim 1, furthercomprising an external polymer coating that overlies the solidelectrolyte, wherein the external polymer coating contains a pluralityof pre-polymerized conductive polymer particles.
 10. The solidelectrolytic capacitor of claim 9, wherein the external polymer coatingcontains a first layer that overlies the solid electrolyte and a secondlayer that overlies the first layer, wherein the first layer contains acrosslinking agent and the second layer contains the pre-polymerizedconductive polymer particles.
 11. The solid electrolytic capacitor ofclaim 1, further comprising an anode termination that is electricallyconnected to the anode and a cathode termination that is electricallyconnected to the solid electrolyte.
 12. The solid electrolytic capacitorof claim 1, wherein the capacitor exhibits an equivalence seriesresistance of about 100 mohms or less at a temperature of −55° C. and anoperating frequency of 100 kHz.
 13. A method for forming a solidelectrolytic capacitor, the method comprising: anodically oxidizing asintered porous anode to form a dielectric layer that overlies theanode; forming a first layer of a solid electrolyte over the dielectriclayer by a process that includes chemically polymerizing a monomer insitu to form a conductive polymer, and thereafter forming a second layerthat overlies the first layer by applying a dispersion that includes aplurality of pre-polymerized conductive polymer particles and a separatehydroxy-functional nonionic polymer.
 14. The method of claim 13, whereinthe in situ polymerized conductive polymer and the pre-polymerizedconductive polymer particles include is a substituted polythiophene. 15.The method of claim 13, wherein the hydroxy-functional nonionic polymerhas a molecular weight of from about 300 to about 1,200 grams per mole.16. The method of claim 13, wherein the hydroxy-functional nonionicpolymer is a polyalkylene ether.
 17. The solid electrolytic capacitor ofclaim 1, wherein the hydroxy-functional nonionic polymer constitutesabout 2 wt. % or less of the solid electrolyte.
 18. The solidelectrolytic capacitor of claim 1, wherein the concentration ofhydroxy-functional nonionic polymers in the first layer is about 2 wt. %or less.